The ammonothermal method is a method for producing a desired material by using an ammonia solvent in a supercritical state and/or a subcritical state and utilizing the dissolution-precipitation reaction of a raw material. In applying the ammonothermal method to crystal growth, a supersaturated state due to a temperature difference is generated by making use of the temperature dependency of the solubility of a raw material in an ammonia solvent, whereby a crystal is precipitated.
In a hydrothermal method that is similar to the ammonothermal method, crystal growth is performed by using water in a supercritical state and/or a subcritical state for the solvent, and this method is applied mainly to an oxide crystal such as quartz (SiO2) and zinc oxide (ZnO).
On the other hand, the ammonothermal method can be applied to a nitride crystal and is being utilized for the growth of a nitride crystal such as gallium nitride. Growth of a gallium nitride crystal by the ammonothermal method is a reaction in a supercritical ammonia environment at a high temperature and a high pressure (500° C. or more, 150 MPa or more), and design of the apparatus or selection of the material so as to withstand such an environment is not easy.
Since solubility of gallium nitride in pure ammonia in a supercritical state is extremely low, a mineralizer is added to enhance the solubility and thereby accelerate the crystal growth. The mineralizer is classified into an acidic mineralizer represented by an ammonium halide NH4X (X=Cl, Br or I) and a basic mineralizer represented by an alkali amide XNH2 (X=Li, Na or K). The supercritical ammonia environment containing such a mineralizer is a very harsh corrosive environment.
A pressure vessel (autoclave) can be produced by using a material having strength high enough to withstand the temperature and pressure in the corrosive environment above (for example, Alloy 625 and RENE 41 which are an Ni-based superalloy), but the pressure vessel does not have complete corrosion resistance to supercritical ammonia. In particular, the acidic mineralizer has a strong propensity to corrode the above-described alloy and therefore, it is required to establish a technique for resisting corrosion by using a material having high corrosion resistance.
In this connection, in the case of using an acidic mineralizer, a noble metal (platinum, iridium, and platinum-iridium alloy) confirmed to be resistant to corrosion is used as a material for lining the inner surface of an autoclave or as a material for an inner cylinder-type reaction vessel (Patent Document 1).
In growing a gallium nitride single crystal by the ammonothermal method, a seed crystal is usually used. A crystal having a lattice constant the same as or very close to that of the crystal to be grown is used. A most ideal seed crystal is a gallium nitride single crystal, and a gallium nitride single crystal is obtained by homoepitaxial growth thereon.
However, in practice, a gallium nitride fine crystal is sometimes unintentionally precipitated in a place other than on the seed crystal. This precipitation occurs due to spontaneous nucleus generation. The place other than on the seed crystal means the inner surface of the reaction vessel, that is, the surface of the noble metal lining material or the surface of a seed crystal-holding structure.
The gallium nitride fine crystal due to spontaneous nucleus generation inhibits crystal growth on the seed crystal where a crystal should be precipitated, and this gives rise to reduction in the productivity. Accordingly, for enhancing the productivity of a nitride crystal, it is inevitable to prevent precipitation of a gallium nitride fine crystal in a place other than on the seed crystal as much as possible.
As the method for solving such a problem, there has been proposed a method where a substance differing in the critical density from the solvent ammonia is introduced into the reaction vessel and the substance is unevenly distributed to the top or bottom of the reaction vessel by utilizing a critical density difference, thereby preventing crystal precipitation in those portions (Patent Document 2).
Furthermore, in order to prevent the generated gallium nitride fine crystal from being desorbed and taken into a nitride crystal under growing, it has been proposed to dispose, for example, a precipitate collection net and an umbrella plate for preventing precipitation (Patent Document 3).